The first PC shooter to have a proper story and a good one at that. One of the first games I can remember that maintained the fourth wall and kept you in the game from the POV of Gordon Freeman the whole way through. Also one of the best opening title sequences of all time.
My first home computer was a C64 and this tape was permanently in the deck. For the younger readers old games came on tape and you had to wait sometime up to 30 minutes while it loaded to play a game. I loved this little platformer and I still think I could remember the patterns of the birds around the screen and get through at least the first 10 levels without issue.
Call of Duty 4 Modern Warfare
I loved Call of Duty 1,2 and 3 but it was 4 that blew my mind and stole days of my life to the multiplayer. Great graphics, explosive set pieces and great balancing in the multiplayer made it very compulsive playing and possibly threatened my marriage.
Mass Effect 2
I have never really been in to RPG’s but Mass Effect 2 changed that and opened my eyes to a new genre of gaming. I must admit I tried Mass Effect 1 and didn’t like it so I was surprised when I got so sucked into the second game. Great story telling and characters make this possibly the best sci fi RPG of all time.
Created as a mod to Half Life which put it on my radar straight away. This game I believe was the first game to be played professionally feature televised league matches. It is a perfectly balanced multiplayer experience. Practice is essential to master it, but the time you put in to it is paid back with the satisfaction of a victory against the odds.
This is only my opinion and there maybe games I have forgotten. Let us know what your top 5 games would have be.